1. Field of the Invention
The present invention is directed to utilizing measured X-Y deviations in assemblywise enthalpy rises, determined using incore thermocouples measuring the temperature of coolant exiting selected fuel assemblies, from established reference values and correlating the enthalpy rise deviations with rod control cluster axial positions in a nuclear reactor core to provide an on-line thermocouple based rod position indication system for a pressurized water nuclear reactor.
2. Description of the Related Art
Knowing the position of control rods in a nuclear core of a nuclear reactor is essential to the safe operation of a nuclear power plant as well as a legal requirement for continuation of an operating license. Currently rod position is determined using a coil stack that sits on top of the containment vessel in which the control rod drive shaft moves up and down. The magnetic impedance produced voltage output of the coil stack is proportional to the length of the rod drive shaft residing in the coil stack, thereby allowing rod position to be inferred through the voltage output of each coil stack. This type system is susceptible to calibration drift requiring periodic recalibration even when the rods are not moved. To determine the accuracy of this magnetic rod position inference system or analog rod position indication system and to recalibrate same, a movable incore detector system must be periodically used to detect the change in flux produced within the reactor caused by the damping action of the rod and to compare this approximate rod position, inferred using the incore detectors, with the rod position indicated by the magnetic inference system. Operating this incore detector system requires exposure of maintenance personnel to small amounts of radiation. This system also requires that a movable detector string be sufficiently close to the target rod to detect flux changes caused by the target rod. If rod position of the target rod cannot be verified, the rod must be considered inoperable. A reactor shutdown is necessary if more than one rod is considered inoperable.